What would be your best app for learning a language?
What I have found successful (aka maximizes my development) is when I am watching the videos and only about one out of every thirty words is a challenge to understand from context.
If you can listen to them with a partner, it's even more efficient, because you can practice pronunciation together. It's been two years since we listened to Japanese bits (trip was cancelled), but we still joke about the words we remember.
(I also use Duolingo, but you have to very consciously make an effort to learn. And I'm mostly just doing it for fun and exposure to languages)